Such metallic materials as steel sheets plated with a zinc-containing metal, aluminum sheets and the like are used in broad fields such as automobiles, building materials and appliances. However, the zinc and aluminum used in these metallic materials are corroded in ambient air to form a corrosion product which is the so-called white rust. This corrosion product disadvantageously impairs the appearance of metallic materials and further detrimentally effects the adhesiveness of the paint coating.
Therefore, in order to improve corrosion-resistance and adhesiveness of paint coating, chromating is usually applied on the surface of metallic materials by using a treating agent which contains as the main components chromic acid, bichromic acid or its salts.
Nevertheless, chromating is liable to often avoided because consciousness of environmental conservation is recently intensified. The hexavalent chromium contained in the chromating liquid used for treating the surface of metallic materials exerts detrimental influence directly upon the human body. In addition, the waste water, which contains hexavalent chromium, must be subjected to special treatment as stipulated under the Law of Prevention of Water Pollution. This is a reason that the total cost of the surface-treated metallic materials is considerably increased. In addition, when the equipment, in which the chromated metallic material is used, is discarded, the metallic material becomes industrial waste which, however, contains chromium. Although these materials are chromium-source, a serious disadvantage is that they cannot be recycled. This involves a socially serious problem.
Meanwhile, a well-known surface treating method other than chromating is that using a surface-treating agent which contains tannic acid which, in turn, contains polyphenol carboxylic acid. When the metallic material is treated with an aqueous solution of tannic acid, a protective film is formed by the reaction between the tannic acid and the metallic material. It is believed that since the protective film acts as a barrier against intrusion of corrosive material, the corrosion-resistance is enhanced.
However, since, recently, high corrosion-resistance of the film per se is required along with quality enhancement of the products, the corrosion-resistance attained by the tannic acid alone or the tannic acid together with an inorganic component falls short. It is not possible for such film, therefore, to be put to practical use under the present situation.
There is a disclosure in Japanese Unexamined Patent Publication (kokai) No. Sho 53-121034 to enhance the corrosion resistance, that is, an aqueous solution, which contains water dispersive silica, alkyd resin and trialkoxy silane compound, is applied on the metal surface and dried to form the coating film.
There are disclosure in Japanese Unexamined Patent Publications (kokai) Nos. Sho 57-44,751 and Hei 1-177,380 of a surface treatment method, in which a water-soluble resin consisting of a hydroxypyrone-compound derivative is used to impart to the metallic material the corrosion-resistance, and a method, in which a water-soluble or water-dispersive polymer of a hydroxystyrene compound is used to impart to the metallic material the corrosion resistance.
However, none of the above-described methods can form a film which has sufficiently high corrosion-resistance capable of substituting for the chromate film. The practical issue is, therefore, that the above-described problems have remain unsolved. Accordingly, it is strongly required under the present situation that the non-chromate surface-treatment agent and the treatment method be developed for the metallic material to improve corrosion-resistance.
It is an object of the present invention to solve the above described problems involved in the prior art, and to provide a non-chromate type surface-treatment agent for metallic materials, which agent can form on the surface of metallic material a film having improved corrosion-resistance, and which agent can improve fingerprint resistance, blackening resistance and adhesiveness of the paint coating. It is also an object of the present invention to provide a surface-treating method using the non-chromate type surface-treatment agent.